850 
GEORGE C. PRICE 
in which the head kidneys were less compUcated than in the adult, 
and he was thus enabled to settle conclusively certain disputed 
points. His paper contains an excellent review of the literature 
of the subject. 
The present work has been done on Bdellostoma stouti. This is 
the only myxinoid in which the development of the excretory 
organs is at present known (Price '97, '04). and hence it offers 
a peculiar advantage for the study of the adult organs, as their 
somewhat complicated structure may be interpreted in the light 
of embryology. This, together with the discovery of a probable 
function of the head kidney, is the excuse for the present paper. 
With the exception of a very few species of bony fishes, the my\- 
inoids are the only Craniota described as having a persistent head 
kidney. As was pointed out by Johannes Miiller, and as may 
readily be seen by a glance at one of his figures, which has been 
widely copied in text-books of comparative anatomy and embry- 
ology, the functional kidney in the myxinoids is very simple, much 
simpler than in any other of the Craniota; in fact it is more like 
the kidney of an embryo than of an adult. From chis, as well as 
from the systematic position of the group, one might expect that 
the head kidney would likewise be simple, and that a study of the 
excretory organs of the adult would throw light on the question 
of the homology of the pronephros and mesonephros. But this 
has not proved to be the case; the head kidney is less simple than 
the functional kidney, and the primitive relations between them, 
which is clear in the embryo, is lost in the adult. A study of their 
development proves conclusively that these two organs are ho- 
mologous, but this is the only way in which one can be sure of the 
fact. 
In the embryo the excretory organs arise as a series of segmen- 
tally arranged tubules, opening into the coelon, and extending 
in the specimens studied from segments 11-13 to segments 79-82, 
the exact point both of beginning and of ending varying in difi'er- 
ent individuals and also on the two sides in the same individual. 
The excretory duct appears later than the tubules and arises from 
them. Thus it w^U be seen that in its origin the entire organ has 
the characteristics of a pronephros. One pronephric character- 
